The present invention relates to an automatic spraying apparatus used for forming hard polyurethane foam coatings.
Hard polyurethane foam, which is one of the foamed plastics used as the heat insulating material, is known to have some peculiar properties that it has a very low thermal conductivity, that it may be foamed at any desired place and that it exhibits the strong adhesion to the surface to be coated.
Polyurethane foaming methods are divided roughly into two methods of the pouring method in which polyurethane foam feed stock is poured into a mold or the like and the spraying method in which the feed stock is directly sprayed upon the surface to be coated so that the layer of the hard polyurethane foam coating may be formed. Since the spraying method does not use the molds or the like, it is very advantageous and simple when used for forming the heat insulating layers of a relatively thin thickness of large storage tanks for storing the high and low temperature liquid, cold storage warehouses, etc. In USA, the spraying method is also widely used for heat insulating the roofs.
The spraying apparatus used for forming hard polyurethane coatings is substantially similar in principle to that used for spraying paint or the like, and comprises storage tanks for storing polyurethane foam feed stock, pumps, pipes or hoses and a spray gun. In general, the hard polyurethane foam feed stock consists of two feed stock components in the form of liquid and they are solidified in a moment after being mixed with each other. For this reason, two storage tanks, two pumps and two feed or supply lines are provided in the spraying apparatus and the two feed stock components must be mixed with each other immediately before or after they are sprayed by the spray gun. In some cases, the hard polyurethane foam feed stock consists of three liquid components.
The spraying method is relatively simple in principle as described above, but since the foaming, which is very complex both physically and chemically, takes place on the surface to be treated, the ratio of flow rates of the two liquid feed stock components as well as their temperature must be strictly controlled. Furthermore the conventional manual spraying method has the problems that the thickness of the coating layer must be precisely controlled only by the skilled and experienced workers and that the working conditions are harmful to the workers because the mist of the feed stock is sprayed.
In the extensive studies and experiments conducted by the inventors for developing the satisfactory method for insulating the liquefied natural gas storage tanks, they found out that the hard polyurethane foam coatings formed by the spraying method are more smooth and uniform than those formed by the pouring method using the molds or the like and are free from molding distortions so that they are best adapted for use as the heat insulating material for low temperature liquid. However, the surfaces of the storage tanks to be treated are very large in size and the spraying operations must be strictly controlled so that a novel spraying apparatus must be devised.